In olefin polymerizations, the polymer melt flow is controlled by varying the hydrogen/propylene ratios in the reactor. At present, changes in polymer melt flow are accomplished by changing the hydrogen concentrations in the reactor. Traditionally, melt flow transitions from low melt flow to high melt flow were accomplished by, quickly adding hydrogen into the reactor. Similarly, to achieve a melt flow transition from high melt flow to low melt flow, extensive venting of the reactor has been traditionally required to reduce the hydrogen concentration of the material contained in the reactor. This venting of the reactor to obtain hydrogen concentrations at the desired level can take many hours and cause expensive production delays. For example, using conventional melt flow transition techniques and propylene as the polyolefin in the polymerization reaction, to reduce melt flow from high to low, venting of the reactor can require up to 40 hours at a vent rate of 6000 lbs/hr propylene to reduce melt flow from 50 to 3.
Transition from a higher melt flow to a lower one is dependent upon reactor residence time, reactor design, and the magnitude of the change in the melt flow value. The time to effect a high melt flow to low melt transition by venting can vary from minutes to many hours depending upon the three above-noted parameters. To overcome this extremely costly method which has a tendency to cause production delays, a novel process for adjusting polymer melt flow from high to low has been developed. Practice of this novel invention results in a high melt flow to low melt flow transition time which takes less than about 50% and even as little as about 10% to about 1% of the conventional transition time previously required for conventional venting.